The nature and extent of genetic variation in natural populations and its adaptive significance are central questions in population genetics. The proposed work will address these questions by (1) assessing the extent of variation in DNA sequences from structural and flanking regions of the esterase-6 gene from Drosophila melanogaster and (2) determining the nature of DNA sequence differences associated with variation in the expression of the esterase-6 gene in species related to D. melanogaster. A previously isolated, recombinant DNA clone of the esterase-6 gene and its flanking regions will be sequenced, and the location of sites corresponding to the initiation and termination of transcription will be determined. The molecular basis for the null phenotype of an esterase-6 allele which contains an insertion of a transposable element will be determined. Variation in two natural populations of D. melanogaster will be assessed using restriction site analysis. Sequences homologous to esterase-6 will be isolated from related Drosophila species and sequenced. The identity of sequences involved in the regulation of the esterase-6 gene will be identified by P-element mediated transformation of recombinant constructs of the gene. These data will be used to assess the adaptive significance of allozyme variation at the esterase-6 locus and to construct hypotheses about evolutionary changes in sequences which regulate the expression of structural genes.